Moses had been on Mt. Sinai forty days and forty nights, talking to God and getting instruction. The children of Israel must have thought that Moses was not coming back to them because he had been gone so long. They more than likely assumed that Moses had died or had abandoned them. Therefore, the children of Israel demanded that Aaron make them a god to worship.
In doing this, the children of Israel had disobeyed God. They fashioned a calf made of gold and had worshiped to it, sacrificed to it, and had called the golden image, their god. According to God, these were stubborn people. God was going to take out His wrath on these people by destroying them with disease. He was going to disinherit them. He was going to start over with Moses, making him a nation greater and mightier than they.
Moses pleaded with the Lord on behalf of the children of Israel. Moses acknowledged to God that he understood God's anger, but pointed out that He had brought these people out of Egypt with great power and with a mighty hand. He explained to God that the Egyptians would think that God saved His people just to harm and destroy them. Moses convinced God to not harm His people. God relented from the harm which He said he would do to His people. Moses was courageous and he had a caring heart. He stood between God and the children of Israel. He saved their lives and they didn't even know it. God respected Moses enough to listen to him and heed his advice. This says a lot about the character of God, and the honor and loyalty of Moses.
Idolatry makes God angry. He is a jealous God. He does not want anything put before Him. We are not to put anything before our love for God. Family, friends, money, possessions, careers. Nothing. Idols are anything that takes over and consumes our lives. God wants to be first in our lives and if He isn't, it makes Him angry. We should learn from the children of Israel. They were almost destroyed by their impatience and stubbornness. Wait on the Lord. He is in control of all things, and He will work all things together for good.